2 a symbol in a logical or mathematical expression that can be replaced by the name of any member of specified set
that which holds, denotes or reserves a place for something to come later
A placeholder is a term, sign or some other thing which generally stands in the place of a term, etc., with a specific meaning.
In disciplines involving formal languages, particularly mathematics and logic, the placeholder concept is replaced by the more carefully defined concepts of free variables and bound variables. In computer science placeholders are sometimes referred to as metasyntactic variables.
In politics, a placeholder is sometimes used to indicate that a person is acting in a position on a non-permanent basis or as a proxy.
placeholder in French: Marque substitutive